Device for monitoring and controlling evaporation rate in vacuum deposition



mans "3.1.419

Feb. 2, 1965 DEVICE FOR MONITORING AND CONTROLLING EVAPORATION RATE IN VACUUM DEPOSITION P. R. PAYNE. JR

Filed March 27 1962 POWER POWER POWER SUPPLY SUPPLY SUPPLY INVENTOR. PA UL R.PAY N EJR.

ATTORNEY licited rates DEVICE FOR MONITORING AND CGNTROLLING EVAPORATION RATE ZN VACUUM DEPQSZTEGN Paul R. Iayne, in, Boston, Mass, assiguor to Ailoyd Electronics, Cambridge, Mass, a corporation of Massachusetts Filed Mar. 27, 1962, Ser. No. 182,899

Claims. (Cl 118-7) The present invention relates to the deposition of thin films by evaporation at low pressure and, more PatilCLl larly, to the monitoring and controlling of the rate of Ali rate is a strong function of source temperature, it can-- not be elfectively controlled by monitoring input power or source temperature.

The primary object of the present invention is to monitor the evaporation rate by a novel detection and control system in which part of the vapor stream is ionized. These ions are collected so as to provide an electrical signal and this electrical signal serves to control the heat applied to the source material. In a preferred system. the vapor stream is ionized by an electron beam and the source material is heated by an electron beam.

Other objects of the present invention will in part be f} obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the features, properties and relation of components that are exemplified in the following detailed disclosure, the scope, of which will be indicated in the appended claims.

For a fuller understanding of the iature and objects of the present invention, reference should be had to the following detailed description, taken in connection with the accompanying drawing, which is a single figure in the form of a diagrammatic view, partly mechanical and partly electrical, or" a preferred embodiment of the present invention.

Generally, the illustrated embodiment of the present invention includes an assembly having an evaporator section it} and a detector section 12 disposed in a suitable evacuated chamber 34. Evaporator section it) includes. a crucible 16 for the reception of a sou'ce material. Crucible 36, which is electrically conducting, serves as an anode for the reception of electrons from an annular filament 18 serving as a cathode. The electron flow from filament 18 to crucible 16 heats crucible 16 and consequently causes a flow of vapor 20 from crucible 16 to a substrate 22 through detector section 12. Detector section includes two communicating ionization chambers 24 and 26. Chamber 26 is in the path of the vapor fiow from crucible 16. Chamber 24 is outside the path of the vapor flow from crucible 16. A selfv accelerated electron gun 27 directs an electron beam through chambers 24 and 26 in such a way as to ionize the vapor in chamber 24 and the vapor in chamber 26. Chambers 24 and 26 include ion collectors 28 and 30 respectively. The ion current from within chamber 26 to collector 30 is a function of evaporation rate and background gas pressure. The ion current from within chamber 24 to collector 28 is a function of background gas pressure only. The difiierence between the two ion currents is proportional to evaporation rate.

The ion current of chamber 26, which is composed of both residual gas ions and evaporating source material ions, is relatei to the ionizing electron current I as follows:

1 :1 (2 v"vLn: e BB a) where N.,= No. of vapor molecules in chamber 26 per cu. cm. a =I0niZati0n cross-section of vapor in chamber 26 in cmF. L =Path length of electrons through vapor N =No. of vapor molecules in chamber 24 per cu. cm. rr =ionization cross-section of vapor in chamber 24 in The ion current I of chamber 24, which is composed of residual gas ions only, is similarly related to the ionizing electron current as follows:

The difierence between the two currents is proportional only to the vaporization rate thus.

I=I112 a (1'e since e is very small, L; can be rewritten it u v 'v e but N P /KT where: P =vapor pressure T =crucible temperature, and K=Boltzmanns constant Therefore '7 KT Since the temperature does not vary appreciably over wide ranges of P rr /KT can be considered a source material constant C and I L can be considered a system constant C so that P =C C I Since the evaporation rate W is a known function of 3,, =f 3) In the illustrated form of the assembly, crucible 16 and annular filament 18 are shown as being insulatedly mounted within an open ended conducting shield 32. A power supply 34 generates between annular filament l8 and crucible 16 a voltage of the order of 3 KY. and a current of the order of 300 ma. The precise temperature of annular filament 18, the emission of which is temperature limited, is determined by a power supply as which adjustably delivers a voltage of the order of. 10 v. and a current of the order of 10 21.

Chambers 24 and 26 are shown as being defined by conducting shields 38 and 40, which are insulatedly mounted within housing 14. Shields 38 and 48 are open ended along the axis of election beam 42 from gun 27. Shield 40 provides a path for the flow of vapor from crucible 16. The electron beam path and the vapor flow path provided by shield 40 are at right angles to each other. Electron gun 27 includes an electron emitting filament 43 and a plurality of accelerating electrodes 44, 46 and 48. Filament 43 is heated by a power supply 50 which delivers a voltage of the order or" 10 v. and a current of the order 10 a. The potential difference between filament 43 and the accelerating electrodes is established by a power supply 52 which delivers a voltage of the order of 400 v.

In operation, a high gain operational amplifier 53 is provided for amplifying an input signal that controls the power delivered to filament 18 by power supply 36. 0perational amplifier 53 has a pair of inputs 54 and 56 which respectively are resistively coupled to collectors 28 and 3? of chambers 38 and 40. Error signals amplified by amplifier 52 are in reference to a preset voltage established at 58. Thus, the ion current monitor will measure either input currents or the differential current depending on the mode of operation. The intensity of electron beam 42 is monitored by an operational amplifier 60 which applies a signal to power supply 50 in response to variations in the current from filament 43, a part of which is directed across a resistor 62. The opposed terminals of resistor 62 are applied to the inputs of operational amplifier 60 in order to produce error signals in reference to a present voltage at 64. The pressure within chamber 14 is reduced by a suitable vacuum pumping system 66 to a level below 10- mm. Hg.

The present invention thus provides a direct technique for precisely controlling and measuring evaporation rate in a vacuum deposition system. Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be inter preted in an illustrative and not in a limiting sense.

What is claimed:

1. A system comprising first means for heating a selected source material to generate a stream of vapor, second means for positioning a substrate in said stream, third means for continuously evacuating the region of said stream, fourth means for ionizing a proportion of said stream, fifth means for collecting ions therefrom in order to generate a signal and sixth means for applying said signal to said first means in order to control the temperature thereof.

2. The apparatus of claim 1 wherein said first means includes a crucible within which said source material may be held and a source of electrons between which and the crucible and electron flow may be established.

3. The apparatus of claim 1 wherein said fourth means provides a pair of chambers, seventh means for directing an electron beam through both of said chambers, one of said chambers being positioned in said vapor stream, and eighth means for comparing the ion currents generated within said chambers by said electron beam.

4. A system comprising first means for heating a selected source material to generate a stream of vapor, sec ond means for positioning a substrate in said stream. third means for continuously evacuating the region of said stream, fourth means for ionizing a proportion of said stream, fifth means for collecting ions therefrom in order to generate a signal, Siilil means for applying said signal to said first means in order to control the temperature thereof, said first means includes a crucible within which said source material may be held and a source of electrons between which and said crucible an electron flow may be established, said sixth means controlling the intensity of said electron tlow.

5. The apparatus of claim 4 wherein said fourth means provides a pair of chambers, seventh means for directing an electron beam through both of said chambers, one of said chambers being positioned in said vapor stream, and eighth means for comparing the ion currents generated within said chambers by said electron beam.

6. The apparatus of claim 5 wherein the-ion and current in one of said chambers is perpendicular to said electron beam.

7. A system comprising a housing, a pump for evacuating said housing continuously, an electrically conducting crucible in said housing for reception of a material to be evaporated, a support in said housing for a substrate to be coated with said material when evaporated, said material when evaporated constituting a stream between said crucible and said substrate, a pair of electrical collectors partially bounding a pair of regions, an electron gun for directing an electron beam through said chambers, one or" said chambers being disposed in said stream, the other of said chambers being spaced from said stream, an electron source for directing electrons to said crucible, and a sys- Retereuces Cited in the file of this patent I UNITED STATES PATENTS Kohl July 25, 1956 Morgan Feb. 21, 1961 

1. A SYSTEM COMPRISING FIRST MEANS FOR HEATING A SELECTED SOURCE MATERIAL TO GENERATE A STREAM OF VAPOR, SECOND MEANS FOR POSITIONING A SUBSTRATE IN SAID STREAM, THIRD MEANS FOR CONTINUOUSLY EVACUATING THE REGION OF SAID STREAM, FOURTH MEANS FOR IONIZING A PROPORTIN OF SAID STREAM, FIFTH MEANS FOR COLLECTING IONS THRERFROM IN ORDER TO GENEREATE A SIGNAL AND SIXTH MEANS FOR APPLYING SAID SIGNAL TO SAID FIRST MEANS IN ORDER TO CONTROL THE TEMPERATURE THEREOF. 